1. Field of the Invention
The present invention is related generally to stator coil connections in large turbine-generators, and more particularly to methods and apparatus that improve flashover voltage levels in such turbine-generators by providing a semiconducting layer on the inside surfaces of insulation that surrounds the stator coil connections.
2. Statement of the Prior Art
Dynamoelectric machines (e.g., large turbine-generators) typically comprise a stator with a core, a plurality of coil-receiving slots in the core, and a stator winding wound about the core. The stator core in typical dynamoelectric machines is made up of a plurality of laminations, and stator windings including a plurality of hard-sided coils are frequently used in such stator cores. Because of the irregularities that may be formed between the coils and the coil-receiving slots, the use of semiconducting materials to fill voids that are caused by such irregularities has become a fairly common practice in the prior art for inhibiting corona.
For example, U.S. Pat. No. 4,001,616 (Lonseth et al.) desirably inhibits corona in dynamoelectric machines through the provision of conductive paths from the outer surfaces of each conductor insulating jacket to the magnetic core of the dynamoelectric machine. The coil sides in such machines each consist of a plurality of conductor strands that are insulated one from the other, and that are totally enclosed in a jacket consisting of a number of superposed layers of insulation with an outer armor jacket. As is conventional, the insulating layers are resin bonded micaceous tapes while the outer armor jacket consists of one or more layers of semiconducting tape or paint (i.e., a material having a controlled resistance). When fully cured, the resin bonded micaceous tapes become very hard and unyielding. Moreover, Lonseth et al. teach that even with the most careful application of the tapes and curing of the resins with the coil sides in pressure molds, there is some variation in the width of the coil sides and flatness of their radial surfaces occurs causing voids which can lead to corona. In order to avoid such possibility of corona, Lonseth et al. coat the radial surfaces of each coil side with a thin layer of semiconducting material that will adhere to the outer armor jacket of the coil sides, but which will deform when the coil side is pressed into its slot.
A somewhat similar, but separately insertable conformable side filler is disclosed in U.S. Pat. No. 4,008,409 (Rhudy et al.). Each of the side fillers generally comprise a resin impregnated fiber glass mat having a plurality of ridges formed of a semiconducting, pressure-deformable elastomer on one side thereof. Rhudy et al. suggest that such side fillers not only prevent formation of corona across any voids which might exist between the sides of the coil insulation and the sides of the coil-receiving slots, but also prevent undesirable temperature rises and insulation-destroying vibrations.
Yet another approach which has been used in reducing the incidences of corona in coil slots is disclosed in U.S. Pat. No. 4,473,765 (Butman, Jr. et al.). A grading layer is added between the conventional insulating slot armor elements which line the sides of the coil slots and rotor windings contained therein. Generally consisting of an outer layer of a smooth, hard, tough, flexible material that is capable of withstanding elevated temperatures without charring (e.g., an aramid paper), the grading layer in accordance with Butman, Jr. et al. also includes silicon carbide particles which are bonded in a cured plastic matrix to the insulating slot armor elements and the aramid paper.
The prior art practices noted above provide solutions to the problem of corona which may arise as a result of any voids between the coil sides and the sides of their respective coil-receiving slots in the stator core, but they avoid addressing the problem of corona and its potential for causing flashover in the end turns of such stator windings. Stator windings in conventional turbine-generators typically include a plurality of coil portions which are disposed within the coil-receiving slots, and a plurality of end portions extending axially from the core as extensions of the coil portions.
The end portions of the stator winding are divided along the stator circumference into several alternating groups that belong to different phases of the stator winding. Every such group comprises several end portions belonging to one winding phase. Accordingly, each end portion has connecting means to electrically couple the stator winding into its predetermined phase groups. Conductors within the stator winding are often stranded, except in the smallest ratings, to enable an easier shaping of the coil and to limit eddy current losses that may result from the flux which crosses the coil-receiving slot.
The effect of this flux is to produce a voltage within the strand which results in circulating currents (e.g., the thinner the strand, the lesser the voltage and the resulting eddy currents). Since such strands ordinarily are connected together at the joints between the coils, there will also be eddy currents circulating between the strands because of the difference in flux linked by various strands. This source of loss can often be reduced sufficiently by the use of more and thinner conductors per coil, with a corresponding increase in the number of parallel circuits. Alternatively, some kind of transposition may be used to control the eddy currents between strands.
It is known to be satisfactory to make a transposition by groups of strands rather than by individual strands, which can be accomplished in the end portions of the stator winding by connecting the upper groups of one coil in series with the lower groups of adjacent coils. Such groups of strands must, however, be insulated from its adjacent groups throughout the coils included in the transposition.
Another form of insulation must also be provided between the connecting means for each group of end portions. That is, while a comparatively low potential difference exists between two adjacent end portions which are connected together in one winding phase (where the interturn voltage of the one winding phase is determinative of this potential difference), the full line voltage of the stator winding can be present between the end portion connections of two adjacent phase groups. A taped structure, or a special insulating box (e.g., a dielectric box as is shown in U.S. Pat. No. 4,385,254--Vakser et al.) can be used to provide such insulation.
None of the patents that are discussed above completely avoids the problems of corona or its attendant potential for causing flashover in the end portions of the stator winding. It has been found by the inventor herein that irregularities in the overall surface of the end portion connections in one phase group can affect the uniformity of an electrical field between the end portion connections of adjacent phase groups. That is, a non-uniform electrical field which can lead to an occurrence of flashover results when voids exist between the irregular surface of the end portion connections of a single phase group and the insulating means surrounding those phase group connections. This is especially so in stator windings having transposition group connections. As a result, spaces between the end portion connections of adjacent phase groups must be increased in order to avoid discharge during service and flashover during high-voltage tests.